To realize optical communication networks having increased speed and capacity, studies have recently been pursued on optical gate switches used to turn on or turn off the output of an optical signal at intervals on the order of a nanosecond (ns). An optical gate switch using a semiconductor optical amplifier (SOA) is a known example of an optical gate switch that controls the output of an optical signal on the order of a nanosecond. A gate switch using an SOA is hereinafter sometimes referred to as an “SOA optical gate switch”.
An SOA optical gate switch applies a voltage from a driver circuit to an SOA to supply a driving current to the SOA, thereby switching the output of an optical signal to either ON or OFF. The SOA is an optical amplifier and therefore has a characteristic in which the gain varies in accordance with the driving current. For this reason, supply of a given driving current determined by a system to an SOA, for example, causes the SOA optical gate switch to output an optical signal amplified with a desired gain. In such an example, when switching the output of an optical signal on the order of a nanosecond, the SOA optical gate switch switches the given driving current to be supplied to the SOA to either ON or OFF at intervals on the order of a nanosecond.
Here the SOA optical gate switch includes the resistance component of the SOA, the inductance of electric wiring from the driver circuit to the SOA, and so on, and therefore is represented as an equivalent circuit of a Linkwitz-Riley (LR) filter circuit. Accordingly, in the SOA optical gate switch, the timing at which a voltage is applied to the SOA is delayed relative to the timing at which a driving current is supplied to the SOA. For this reason, in the SOA optical gate switch, it takes time before the given driving current flows to the SOA even if a voltage whose rise, which is represented by a step function, is steep. It is therefore difficult to switch the output of an optical signal quickly.
In view of such a situation, there is a technique of providing a high-frequency emphasis filter between the driver circuit and the SOA. In such a technique, the driver circuit applies, to the SOA, a higher voltage than a voltage steadily applied to the SOA, when the output of an optical signal is turned on. Hereinafter a voltage steadily applied to the SOA when the output of an optical signal is turned on is sometimes referred to as a “steady voltage”. In cases in which the steady voltage is continuously applied to an SOA, the given driving current determined by the system is to be supplied to the SOA.
According to the aforementioned technique using a high-frequency emphasis filter, when a voltage whose rise is steep is output from a driver circuit, the high-frequency emphasis filter allows high-frequency components to pass therethrough, so that higher voltages than the steady voltage are applied to an SOA. As a result, in an SOA optical gate switch using a high-frequency emphasis filter, upon output of the voltage from the driver circuit, the given driving current is quickly supplied to the SOA. This is considered to enable the output of an optical signal to be switched quickly.
Japanese Unexamined Patent Application Publication No. 2009-55550A discloses a related art example.
The SOA optical gate switch of the related art using a high-frequency emphasis filter, however, has a problem in that an overshoot occurs in a driving current supplied to the SOA. Such a problem will be specifically described with reference to FIG. 8 and FIG. 9. FIG. 8 illustrates an exemplary configuration of an optical-switch driver circuit of the related art.
As illustrated in FIG. 8, an optical-switch driver circuit 90 of the related art includes a pulse generator 91, an operational amplifier 92, a high-frequency emphasis filter 94, and a resistor 95, and is connected to an SOA 93. In the case of turning on the output of an optical signal, the pulse generator 91 applies a voltage to the SOA 93; in the case of turning off the output of an optical signal, the pulse generator 91 does not apply a voltage to the SOA 93. The operational amplifier 92 performs impedance conversion. The output voltage of the operational amplifier 92 is applied, and, as a result, a driving current is supplied to the SOA 93. In response to this driving current, the SOA 93 amplifies the optical signal and outputs the amplified optical signal. The high-frequency emphasis filter 94 is a resistor-capacitor (RC) circuit including a capacitor 94a and a resistor 94b. The high-frequency emphasis filter 94 in such a structure allows the output voltages of high-frequency components of output voltages of the pulse generator 91 to pass therethrough, and reduces the output voltages of the low-frequency components.
With reference to FIG. 9, an operational example of the optical-switch driver circuit 90 illustrated in FIG. 8 will be described. FIG. 9 illustrates an example of the voltage output by the pulse generator illustrated in FIG. 8. Note that the top of FIG. 9 illustrates an example of the voltage output by the pulse generator 91 illustrated in FIG. 8. The middle of FIG. 9 illustrates an example of the voltage applied to the SOA 93. The bottom of FIG. 9 illustrates an example of the driving current supplied to the SOA 93.
In an example illustrated in FIG. 9, the pulse generator 91 generates pulse waves that rise from 0 V to a constant voltage ‘V0’. Note that it is assumed that the output voltage of the operational amplifier 92 to which the voltage ‘V0’ has been applied is higher than the steady voltage. Here the high-frequency emphasis filter 94 allows the high-frequency component to pass therethrough and therefore outputs the voltage ‘V0’ to the operational amplifier 92 at the instant when the voltage ‘V0’ is applied at time t11 by the pulse generator 91. As illustrated in the middle of FIG. 9, the operational amplifier 92 amplifies the voltage ‘V0’ output from the high-frequency emphasis filter 94, and applies the amplified voltage ‘V11’ to the SOA 93 at the time t11. Note that, in the case in which the operational amplifier 92 has a gain of a factor of 1, the voltage ‘V0’ and the voltage ‘V11’ are approximately the same.
Subsequently, as exemplarily illustrated at the top of FIG. 9, when the output voltage of the pulse generator 91 reaches the constant voltage ‘V0’, the high-frequency emphasis filter 94 gradually decreases the voltage to be output to the operational amplifier 92 from ‘V0’. Accordingly, the voltage applied to the SOA 93 by the operational amplifier 92 is gradually decreased from ‘V11’. In the example illustrated at the middle of FIG. 9, the voltage applied to the SOA 93 by the operational amplifier 92 is decreased from ‘V11’ to ‘V12’ and becomes constant at ‘V12’.
In such a manner, when turning on the output of an optical signal, the optical-switch driver circuit 90 applies the voltage ‘V11’, which is higher than the steady voltage ‘V12’, to the SOA 93 as illustrated at the middle of FIG. 9. As a result, the optical-switch driver circuit 90 quickly supplies a given driving current ‘It’ determined by the system to the SOA 93 as illustrated at the bottom of FIG. 9.
Here the resistance component of the SOA 93 largely varies if the applied voltage exceeds a given value. Accordingly, when a higher voltage than the steady voltage is continuously applied to the SOA 93, a larger driving current than the driving current ‘It’ is supplied to the SOA 93, thereby causing an overshoot, as illustrated at the bottom of FIG. 9. In the example illustrated in FIG. 9, a higher voltage than the steady voltage ‘V12’ is continuously applied to the SOA 93 from the time t11 to time t12. As a result, a larger driving current than the driving current ‘It’ is supplied to the SOA 93 from time t21 to time t22. This leads to a problem in that the gain of the SOA 93 varies and therefore the power of an optical signal output by the SOA 93 varies.